The present invention relates to a process for isomerizing a light hydrocarbon oil. More particularly, the present invention relates to a process for isomerizing a light hydrocarbon oil with a simpler equipment and more economically than in conventional processes.
Isomerization of light hydrocarbon oils is an important technique in organic synthesis chemistry which has conventionally been used frequently in the petroleum refining industry and petrochemical industry. For example, the technique is used for isomerizing n-paraffin which has a straight chain and is liquid at ordinary temperature to isoparaffin, having a side chain, to thereby produce a high-octane-number blending base necessary for improving the octane number of fuel oils for motor vehicles and aircraft. The technique is further used for isomerizing a cyclic compound, such as methylcyclopentane or the like, to cyclohexane, which is effectively utilized as a feed in the organic industry.
In particular, as a result of performance advancement in automotive and aircraft engines, the gasolines for use as fuels therein are required to have a high octane number. Conventional processes for improving the octane number of a gasoline include a process of adding a lead compound octane number improver and a process of blending a large amount of a blending material having a high octane number, e.g., an aromatic hydrocarbon.
However, the addition of a leadoctane number improver has been prohibited because it causes the problem of air pollution, while use of a gasoline having a high aromatic hydrocarbon concentration results in a high aromatic hydrocarbon concentration in the exhaust gas discharged into the air.
Improving octane number by isomerization is hence becoming important more and more. Incidentally, improving octane number by isomerization is not easy technically. For example, it has technically been extremely difficult to improve a gasoline base having an octane number of 80 so that its octane number is elevated by only 1 to 81.
Accordingly, many investigations have conventionally been made on processes for isomerizing a light hydrocarbon oil, and various catalysts for use in the reaction are disclosed. The most useful isomerization catalysts among these include solid super strong acid catalysts.
The solid super strong acid catalysts including those composed of Pt/ZrO2/SO4 have various advantages. For example, these catalysts have a high isomerization activity at a reaction temperature of 400xc2x0 C. or less and can be easily prepared and handled. Furthermore, since they are relatively highly durable in the presence of water and the like, pretreatments, such as dehydration and the like, can be omitted in isomerizing light hydrocarbon oils.
However, there has been a problem that when a light hydrocarbon oil which has not undergone the so-called hydrogenation treatment wherein desulfurization is conducted by hydrogenation and which contains a sulfur compound at a high concentration is passed through a layer of a solid super strong acid catalyst, then the solid super strong acid catalyst is poisoned by sulfur and is deprived of its catalytic activity in a relatively short time period.
Therefore, it is difficult for a light hydrocarbon oil containing sulfur at a high concentration to be directly isomerized without being hydrogenated.
A conventional process for isomerizing a light hydrocarbon oil is hence conducted in the manner shown in FIG. 5. First, a light hydrocarbon oil as a feed oil is subjected to a hydrogenation treatment. In the hydrogenation treatment, pressurized hydrogen gas is mixed with the pressurized light hydrocarbon oil and the mixture is heated to a reaction temperature with a heating furnace 32. The light hydrocarbon oil is passed through a hydrogenation reaction column 34 having a hydrogenation reaction catalyst layer disposed therein. Thus, the light hydrocarbon oil is hydrogenated to convert sulfur compounds contained in the light hydrocarbon oil into hydrogen sulfide.
Subsequently, the light hydrocarbon oil containing hydrogen sulfide formed in the hydrogenation reaction column 34 is cooled with a heat exchanger or cooler 36 and sent to a gas/liquid separation vessel 38, where the light hydrocarbon oil is separated from the hydrogen sulfide. The hydrogen sulfide is removed. During the hydrogen sulfide removal, most of the hydrogen gas is also separated from the light hydrocarbon oil.
The light hydrocarbon oil which has come out of the gas/liquid separation vessel 38 is then distilled in a distillation column 40 to separate light gas components. The light hydrocarbon oil coming out of the bottom of the distillation column 40 is used as a feed oil for an isomerization step.
Furthermore, the light hydrocarbon oil obtained through the bottom of the distillation column 40 is pressurized and hydrogen gas is forced thereinto. This mixture fluid is heated to a reaction temperature with the heating furnace 42. The light hydrocarbon oil as a feed oil is passed through an isomerization column 44 having a layer of a solid super strong acid catalyst disposed therein to isomerize the light hydrocarbon oil.
Conditions for the conventional isomerization step include a pressure of 30 kg/cm2g, an H2/oil ratio of 370 liter/liter, and a reaction temperature of 185xc2x0 C. or more.
However, the conventional process for isomerizing a light hydrocarbon oil is complicated because not only the step of conversion to hydrogen sulfide through hydrogenation treatment but also the step of separating and removing the hydrogen sulfide and the step in which the light hydrocarbon oil which has been separated from the hydrogen sulfide is distilled to prepare a feed oil are conducted as pretreatments for the isomerization treatment of the light hydrocarbon oil. Consequently, the conventional process has had the following problems.
1) A large-scale equipment including, e.g., a gas/liquid separation vessel and a distillation column is necessary for conducting the step of separating and removing hydrogen sulfide and the step of preparing a feed oil. Furthermore, operation of the equipment necessitates personnel and utilities, such as steam, cooling water, and the like. As a result, the equipment cost and operating cost are high and, hence, the cost of isomerizing a light hydrocarbon oil is increased.
2) Due to the step of separating and removing hydrogen sulfide is conducted, it is necessary to temporarily cool the light hydrocarbon oil which has come out of the hydrogenation reaction column and contains hydrogen sulfide. As a result, the step of isomerizing the light hydrocarbon oil necessitates heating the feed oil to a given isomerization temperature and thereby consumes a large amount of thermal energy.
3) Still another problem is that since the separation and removal of hydrogen sulfide is accompanied with separation of hydrogen gas, it is necessary to freshly force hydrogen gas into the light hydrocarbon oil in the isomerization step, resulting in an increase in the cost of hydrogen gas.
An object of the present invention is to provide a process for isomerizing a light hydrocarbon oil with a simpler equipment and more economically than in conventional processes.
As a result of various experiments, the present inventors have found that hydrogen sulfide does not act as a catalyst poison on solid super strong acid catalysts when isomerization is conducted at a temperature higher by at least about 5 to 20xc2x0 C. than the conventional isomerization temperatures, although the temperature cannot be unconditionally specified because it depends on the catalysts. The process of the present invention has thus been completed.
The present invention relates to a process for isomerizing a light hydrocarbon oil which comprises:
subjecting the light hydrocarbon oil to a hydrogenation treatment for converting an organosulfur component in the light hydrocarbon oil into hydrogen sulfide to give an organosulfur content of from 0 to 30 mass ppm as a pretreatment step; and
isomerizing the light hydrocarbon oil still containing the converted hydrogen sulfide in the presence of a solid super strong acid catalyst under conditions at a temperature of from 190 to 300xc2x0 C., a pressure of from 1 to 50 kg/cm2g, an H2/oil ratio of from 180 to 1800 liter/liter, and an LHSV of from 2 to 10 hrxe2x88x921 as an isomerization step.